The invention relates to a semiconductor device comprising a semiconductor body including an integrated circuit provided with bonding pads for connecting the circuit to external supply conductors and with means for protecting the circuit against damage caused by electrostatic discharge, said means comprising at least a protection diode which is connected to a bonding pad and formed by a drain region of a field effect transistor with an insulated gate electrode, which field effect transistor includes a source region and a drain region, which are separated from each other by an intermediate channel region, said source and drain regions being formed by surface areas of a first conductivity type in a surface area of the second, opposite, conductivity type.
Such a device is disclosed, for example, in U.S. Pat. No. 5,493,142.
The use of a field effect transistor as a protection against ESD is generally known. In a much used configuration, often referred to as ggNMOST (grounded gate NMOST), the gate or gate electrode is connected, with its source region, to the bonding pad which is connected to earth or V.sub.ss, while the drain is connected to an I/O pad. If, for example as a result of contact of the pad with a human body, there is a risk that the voltage on the bonding pad will increase to a very high value which may seriously damage or even destruct the circuit, the protection diode will break down before the damage has occurred, thus enabling the electric charge to be removed. The operation of the protection diode is enhanced by the "snap-back" effect, whereby the lateral bipolar transistor, formed by the drain region as the collector and the source region and the surface region as, respectively, the emitter and the base, becomes operative.
To ensure a proper operation of the circuit, it is very important that the properties of the protection element, during operation, are not or hardly subject to change as a result of breakdown. An important parameter in this respect is the leakage current of the diode, because too large a leakage current of the protection diode adversely affects the quality of the entire circuit. In practice it has been found that this leakage current can increase considerably. The robustness of the protection element can be increased by providing a resistance in series with the drain, as proposed in the above-mentioned patent U.S. Pat. No. 5,493,142. The resistance provides for a uniform current distribution over the surface of the diode, so that the formation of generation centers as a result of local current concentration can be precluded. A drawback of such a resistance is, inter alia, that during normal operation the current passing through the transistor also runs through the resistance. In order to keep the overall resistance at a sufficiently low level, it is often necessary to reduce the transistor resistance by increasing the transistor, which requires additional space.